Improper Execution, Generally
Every testator must be at least eighteen years of age to make a valid will in Pennsylvania. Any will executed by someone less than 18 has been improperly executed and is invalid.

Other causes of improper execution are associated with how the will was signed. For instance, a will that has been signed by mark must have two subscribing witnesses, as does a will that another person has signed for the testator.

Other circumstances, such as forgery also create an improper execution that invalidates the will.

See: Signing By Mark; Signing By Another Person; will Contests, Generally